1. Field of the Invention
The present invention relates to an exhaust emission control device for an internal combustion engine, for purifying NOx in exhaust gases flowing through an exhaust passage, by a selective reduction catalyst in the presence of a reducing agent.
2. Description of the Related Art
Conventionally, as an exhaust emission control device of this kind, one disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2004-100700 is known. This internal combustion engine is a diesel engine having an exhaust passage, a reducing agent injection valve, an oxidation catalyst, and a selective reduction catalyst are arranged in the exhaust passage in this order from the upstream side to the downstream side. The reducing agent injection valve injects any of urea, ammonia, and diesel fuel as a reducing agent, whereby NOx in exhaust gases is selectively reduced by the selective reduction catalyst in the presence of a reducing agent. Further, the oxidation catalyst is disposed at a location upstream of the selective reduction catalyst so as to make the ratio x between the respective amounts of NO and NO2 in exhaust gases flowing into the selective reduction catalyst equal to 1:1. Furthermore, the exhaust emission control device includes a controller for controlling the amount of reducing agent injected by the reducing agent injection valve, and a NOx concentration sensor for detecting the concentration of NOx on the downstream side of the selective reduction catalyst, and so forth.
In the above exhaust emission control device, the level of NOx (PPM) estimated to be emitted from the engine is calculated according to operating conditions of the engine, and the ratio x between the amount of NO and the amount of NO2 in exhaust gases flowing from the oxidation catalyst into the selective reduction catalyst is calculated based on the level of NOx and the concentration of NOx detected by the NOx concentration sensor, using a predetermined arithmetic expression. Then, based on the ratio x and the level of NOx, the amount of reducing agent injected (more accurately, the amount of reducing agent to be injected) by the reducing agent injection valve is determined by an arithmetic expression derived from a reaction formula of reaction between the reducing agent, and NO and NOx (Paragraph numbers [0030] to [0033]). More specifically, in the exhaust emission control device, the composition of exhaust gases is estimated based on the value of the NOx concentration detected by the NOx concentration sensor and the operating conditions of the engine. Then, based on the estimated composition of exhaust gases, the NOx purification ratio (i.e. NOx reduction ratio) of the selective reduction catalyst is estimated, and the amount of injected reducing agent is feedforward-controlled based on the NOx purification ratio estimated as above.
In general, the NOx purification ratio of the selective reduction catalyst does not depend on the composition of exhaust gases alone but it varies with a change in the temperature of the selective reduction catalyst due to a change in the operating conditions of the engine 3, degradation of the selective reduction catalyst, and variation in NOx-purifying capability between individual selective reduction catalyst units. Further, some selective reduction catalysts have a characteristic of adsorbing ammonia, and when a selective reduction catalyst having such a characteristic is used, according to a change in the amount of ammonia adsorbed thereby, the amount of a reducing agent necessary for decreasing NOx changes, which causes the NOx purification ratio to appear to be changed.
In contrast, according to exhaust emission control device disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2004-100700, the NOx purification ratio is estimated based on the composition of exhaust gases, and the amount of injected reducing agent is simply feedforward-controlled based on the estimation, and hence no inconveniences are caused insofar as the NOx purification ratio of the selective reduction catalyst is determined simply by the composition of exhaust gases. However, when the NOx purification ratio is changed by various factors other than the composition of exhaust gases as described above, the estimation accuracy of the NOx purification ratio is lowered, which makes it impossible to properly control the amount of injected reducing agent. More specifically, it is impossible to properly control the amount of injected reducing agent e.g. when the temperature of the selective reduction catalyst changes with a change in the operating condition of the engine, when the selective reduction catalyst is degraded, when there is a large variation in the NOx-purifying capability between individual selective reduction catalyst units, or when the amount of ammonia adsorbed by a selective reduction catalyst having the ammonia adsorption characteristic changes. As a result, when the amount of the reducing agent injected to the selective reduction catalyst becomes short, the NOx purification ratio is degraded, whereas when the amount of injected reducing agent becomes too large, the amount of ammonia flowing from the selective reduction catalyst increases, which results in increased exhaust emissions.